QAM Symbols Research Articles

Effect of Subcarrier Activation Ratio on the Performance of OFDM-IM over Rayleigh Fading Channel

Orthogonal frequency division multiplexing with index modulation (OFDM-IM) is acquiring increasing attention due to its advantages over classical OFDM. These advantages include superior BER performance and transmit power saving since only a subset of the subcarriers is activated to transmit $M$ -QAM symbols and the remaining subcarriers are turned OFF. The vast majority of the literature studies the OFDM-IM by assuming certain subcarrier activation ratios, $r$ , together with other system parameters without figuring out the possible variation in system performance at other $r$ values. In this letter, the effect of $r$ and $M$ on the spectral efficiency (SE) and BER of OFDM-IM is investigated. An inequality is developed to maximize SE while power saving is maintained. Moreover, the BER performance of an OFDM-IM system exploiting a maximum subcarrier energy detector over multipath Rayleigh channel is analyzed. An approximate average BER expression is derived and the effect of $r$ and $M$ is discussed. Computer simulation has showed the validity of the derived expressions.

A New Filter-Bank Multicarrier System With Two Prototype Filters for QAM Symbols Transmission and Reception

A filter-bank multicarrier-quadrature amplitude modulation (FBMC-QAM) system with two prototype filters for transmitting QAM symbol is proposed, and the orthogonality conditions for the FBMC-QAM system without the intrinsic interference is derived. The proposed transmitter performs the individual filtering for the even and odd-numbered sub-carrier symbols, respectively. In order to satisfy the derived orthogonality conditions, we perform a sub-block wise reverse ordering procedure for the outputs of the odd-numbered sub-carrier filter. Also, this paper shows that the proposed FBMC-QAM system satisfies the suggested orthogonality conditions and can use the multiple-input multiple-output transmission schemes as the conventional orthogonal frequency division multiplexing (OFDM) systems. The signal-to-interference power ratio and the bit error rate (BER) for the proposed FBMC-QAM system are evaluated. Numerical results show that the proposed FBMC-QAM system has almost the same BER performance compared with the FBMC-OQAM and OFDM systems.

Achieving the Near-Capacity of Two-Way Relay Channels With Modulation-Coded Physical-Layer Network Coding

We propose and design a practical modulation-coded (MC) physical-layer network coding (PNC) scheme to approach the capacity limits of Gaussian and fading two-way relay channels (TWRCs). In the proposed scheme, an irregular repeat–accumulate (IRA) MC over $\textrm{GF}(q)$ with the same random coset is employed at two users, which directly maps the message sequences into coded PAM or QAM symbol sequences. The relay chooses appropriate network coding coefficients and computes the associated finite-field linear combinations of the two users' message sequences using an iterative belief propagation algorithm. For a symmetric Gaussian TWRC, we show that, by introducing the same random coset vector at the two users and a time-varying accumulator in the IRA code, the MC-PNC scheme exhibits symmetry and permutation-invariant properties for the soft information distribution of the network-coded message sequence (NCMS). We explore these properties in analyzing the convergence behavior of the scheme and optimizing the MC to approach the capacity limit of a TWRC. For a block fading TWRC, we present a new MC linear PNC scheme and an algorithm used at the relay for computing the NCMS. We demonstrate that our developed schemes achieve near-capacity performance in both Gaussian and Rayleigh fading TWRCs. For example, our designed codes over GF(7) and GF(3) with a code rate of 3/4 are within 1 and 1.2 dB of the TWRC capacity, respectively. Our method can be regarded as a practical embodiment of the notion of compute-and-forward with a good nested lattice code, and it can be applied to a wide range of network configurations.

Mappings from Binary Variables to QAM Symbols and Improvement of Peak Envelope Power of OFDM Systems

This paper establishes a mapping relationship from independent binary variables to QAM symbols. By these proposed mappings square QAM constellation can be produced by binary signals rather than quaternary signals, which is advantageous to operation of those apparatuses driven only by binary signals. As one of applications of these presented mappings, we give an example to verify improvement of upper bounds of peak envelope power in an OFDM communication system employing QAM complementary sequences.

빔 공간 MIMO 통신시스템에서 고레벨 변조 설계와 평가

ESPAR(Electronically Steerable Parasitic Array Radiator) antenna is the technique for overcoming the problems of space limitation and energy efficiency due to the multiple RF-chain. Conventional MIMO system with multiple antenna requires a large number of RF-chain for transmitting the multiple data because it transmits the data in proportion to the number of antenna. Beamspace MIMO system using the ESPAR antenna which has single RF-chain was proposed for solving the problems caused by using the multiple antenna and RF-chain. In this paper, therefore we propose 2x2 beamspace MIMO system using the 16, 64-QAM modulation and evaluate the performance of this system to reveal that it is possible that beamspace MIMO system can use not only PSK modulation but also QAM modulation. We confirm that QAM symbol can be generated by adjusting reactance of parasitic elements and making reactance set and also we confirm that performance of beamspace MIMO system is similar to the conventional MIMO system by transmitting the QAM symbol made by reactance set through the simulation. 논문 14-39A-05-02 The Journal of Korea Information and Communications Society '14-05 Vol.39A No.05 http://dx.doi.org/10.7840/kics.2014.39A.5.220 220 ※ 이 논문은 2013년도 충북대학교 학술연구지원사업의 연구비 지원에 의하여 연구되었습니다. First Author : Chungbuk National University Department of Electronic Engineering, willfriends7@gmail.com, 학생회원 ° Corresponding Author : Chungbuk National University Department of Electronic Engineering, ecomm@cbnu.ac.kr, 정회원 논문번호 : KICS2013-12-526, Received December 10, 2013; Revised April 16, 2014; Accepted May 7, 2014 논문 / 빔 공간 MIMO 통신시스템에서 고레벨 변조 설계와 평가

이중 편파 MIMO를 쓰는 DVB-T2 시스템의 비트 매퍼 성능 분석

초고해상도의 비디오와 다채널 오디오를 통한 극사실적인 방송 서비스를 지향하는 UHDTV (Ultra-High Definition TeleVision)가 차세대 방송 표준으로 논의되고 있으나 기존의 지상파 방송 시스템의 데이터 전송률이 UHDTV 방송 서비스의 요구 전송률에 크게 미치지 못하기 때문에 새로운 지상파 방송 시스템의 개발 또는 현재 시스템의 전송률 증가에 대한 연구가 필요한 상황이다. 이를 위해 기존의 DVB-T2 (Digital Video Broadcasting-2nd generation Terrestrial) 시스템에 고차 성상 변조 및 이중 편파 안테나를 이용한 다중 입출력 시스템을 적용하여 데이터 전송량을 증가시키는 연구가 진행되고 있다. 비정규 LDPC (Low-Density Parity Check) 부호를 사용하는 DVB-T2에 기반한 다중 입출력 시스템의 성능을 최적화하기 위해, 다중 입출력 시스템을 위한 최적의 비트 매퍼 설계에 대한 연구가 필요하다. 그러나 현재까지 진행되어 온 비트 매퍼 설계는 단일 입출력 시스템에 대해 국한되어 있다. 따라서 본 논문에서는 다중 입출력 시스템의 위한 최적의 비트 매퍼를 설계하기 위한 사전 연구로, 이중 편파 다중 입출력 시스템의 VND (Variable Node Degree) 분포를 나타내는 파라미터를 새롭게 정의하고 정의된 파라미터에 따른 시스템의 수신 성능을 분석하였다. The UHDTV system, which provides realistic service with ultra-high definite video and multi-channel audio, has been studied as a next generation broadcasting service. Since the conventional digital terrestrial transmission system is not capable to cover the increased transmission data rate of the UHDTV service, there are great necessity of researches about increase of data rate. Accordingly, the researches has been studied to increase the transmission data rate of the DVB-T2 system using dual-polarized MIMO technique and high order modulation. In order to optimize the MIMO DVB-T2 system where irregular LDPC codes are used, it is necessary to study the design of the bit mapper that matches the LDPC code and QAM symbols in MIMO channel. However, the research related to the design of the bit mapper has been limited to the SISO system. Therefore, this paper defines a new parameter that indicates the VND distribution of MIMO DVB-T2 system and performs the performance analysis according to the parameter which will be helpful for designing a MIMO bit mapper.

A Statistical Inter-Cell Interference Model for Uplink Cellular OFDMA Networks Under Log-Normal Shadowing and Rayleigh Fading

The performance of uplink cellular OFDMA networks transmitting multiple QAM symbols per hop is evaluated under log-normal shadowing and Rayleigh fading without resorting to a Gaussian approximation of the inter-cell interference. The derived interference statistic is observed to deviate significantly from the Gaussian distribution which may be exploited to dramatically enhance the network performance. It is also observed that the network performance is highly dependent on the number and the geometry of the QAM symbols transmitted within a hop, especially for large code frame lengths.

16QAM transmission with 52 bits/s/Hz spectral efficiency over transoceanic distance

We transmit 160 x 100 G PDM RZ 16 QAM channels with 5.2 bits/s/Hz spectral efficiency over 6,860 km. There are more than 3 billion 16 QAM symbols, i.e., 12 billion bits, processed in total. Using coded modulation and iterative decoding between a MAP decoder and an LDPC based FEC all channels are decoded with no remaining errors.

Constellation Quantization in Constrained Backhaul Downlink Network MIMO

In this paper we consider a downlink multi-cell scenario where a central processor is connected by a finite-throughput backhaul to base stations (BSs) employing beamforming and QAM constellations. Both the serving BS and auxiliary BSs transmit a signal that combines at the mobile terminal (MT) to provide a QAM symbol while achieving a diversity gain. In order to reduce backhaul occupation, auxiliary BSs receive from the central processor only a quantized version of the QAM symbols to be forwarded to MTs. We formalize the problem of maximizing the network spectral efficiency on air for all the MTs within an area illuminated by the cooperative BSs optimizing a) the QAM constellation size, b) the quantization rate dictated by the finite-throughput backhaul and c) the power allocated by each BS. Since the resulting optimization is a mixed integer programming problem, we investigate a suboptimal solution where the same power is allocated to each MT and obtain a simple iterative algorithm for the rate optimization. Numerical results show that for typical cellular scenarios the suboptimal approach yields a network spectral efficiency close to the theoretical limit of Slepian-Wolf encoding.

QAM equalization and symbol detection in OFDM systems using extreme learning machine

This paper presents a new learning-based framework to jointly solve equalization and symbol detection problems in orthogonal frequency division multiplexing systems with quadrature amplitude modulation. The framework utilizes extreme learning machine (ELM), a recent addition to the class of supervised learning algorithms, to achieve fast training, high performance, and low error rates. The proposed ELM scheme employs infinitely differentiable nonlinear activation functions in least-square solution to learn the channel response, which is the equalization part. In addition to equalization, ELM performs symbol detection. Existing learning-based schemes require an additional symbol slicer for the symbol detection. The proposed framework does not experience training bottleneck imposed by gradient descent–based approaches. Simulation results show that the proposed framework outperforms other learning-based equalizers in terms of symbol error rate and training speeds.

Maximum likelihood detection of spatially multiplexed signals via loosely ordered depth-first sphere decoding

The complexity of depth-first sphere decoders (SDs) is determined by the employed tree search and pruning strategies. Proposed is a new SD approach for maximum-likelihood (ML) detection of spatially multiplexed, high-order, QAM symbols. In contrast to typical ML approaches, the proposed tree traversal skips the computationally intensive requirement of visiting the nodes in ascending order of their partial distances (PDs). Then, a new pruning method efficiently narrows the search space and preserves the ML performance despite the non-ordered tree traversal. This proposed approach results in substantially reduced PD calculations when compared to typical ML SDs and, for high SNRs, the necessary calculations can be reduced down to the number of transmit antennas.

Design of a New 4-Dimensional Constellation-Rotation Modulation Method for DVB-NGH

In this letter, we propose a new 4-dimensional constellation-rotation (CR) modulation method that achieves diversity gain of 4 in Rayleigh fading channels. The proposed scheme consists of two consecutive CR operations for QAM symbols unlike the conventional 2-dimensional CR method based on only one CR operation. Computer simulation results show that the new method exhibits much better performance than the conventional one in terms of code rate and channel erasure ratio.

Improve the Performance of LDPC Coded QAM by Selective Bit Mapping in Terrestrial Broadcasting System

In this paper, we employ selective bit mapping to improve the performance of the LDPC coded QAM scheme for terrestrial DTV broadcasting system. The threshold of message-passing decoding can be considerably lowered by selectively mapping the binary components of LDPC codeword to the positions in the m-tuples to be mapped into 2 <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</i> QAM symbols. In our approach, the mapping pattern is described by bit-mapping polynomials, based on which density evolution can be applied. The optimization algorithm is developed with two implementation concerns, using the Chinese DTMB standard as an example. Numerical results illustrate that our proposed approach can improve the decoding threshold by 0.05 dB to 0.499 dB depending on the code rate and the order of QAM modulation. Simulation results show that the actual BER improvement varies from 0.09 dB to 0.6 dB with different code-modulation combinations in both single-carrier and OFDM modes.

Compact Pulse Shaping for Efficient PAM/QAM Signaling

Compact pulse shaping (CPS) is considered with PAM/QAM signals to improve the performance/bandwidth tradeoff. CPS signals are constructed by targeting a finite impulse response of a pulse shaping filter as opposed to the traditional method of targeting a finite frequency response. CPS QAM signals can be optimally decoded using Viterbi decoding on I and Q channels independently. In the case of non-rectangular constellations, a simple adjustment is presented to correct any invalid QAM symbols that result from independent I and Q decoding. Minimum distance calculations indicate that CPS QAM provides between 2.04 and 5.4 dB of gain, depending on constellation size, over raised cosine filtering, when compared at a given value of the normalized bit bandwidth B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">99</sub> T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> .

Gray 부호화된 QAM 신호를 위한 근사화된 MAP 알고리듬

Abstract In this paper, a new approximated MAP algorithm for soft bit decision from QAM symbols is proposed for Gray Coded QAM signals, based on the Max-Log-MAP and a Gray coded QAM signal can be separated into independent two Gray coded PAM signal, M-PAM on I axis with M symbols and N-PAM on Q axis with N symbols. The Max-Log-MAP used distance comparisons between symbols to get the soft bit decision instead of mathematical exponential or logarithm functions. But in accordance with the increase of the number of symbols, the number of comparisons also increase with high complexity. The proposed algorithm is used with the Euclidean distance and constituted with plain arithmetic functions, thus we can know intuitively that the algorithm has low implementing complexity comparing to conventional ones. Key Words : Soft Decision, Log Likelihood, QAM, Gray Mapping, Max-Log-MAP * 교신저자 : 현광민(kamihyun@gwnu.ac.kr)접수일 09년 10월 04일 수정일 (1차 09년 11월 30일, 2차 09년 12월 11일) 게재확정일 09년 12월 16일

Efficient fast blind equalization with two‐stage single/multilevel modulus and DD algorithm for 64/256/1024qam wired cable communications

In this paper, an efficient blind equalization with two‐stage single/multilevel modulus and decision directed (DD) least mean square (LMS)‐like algorithm is proposed for high‐order QAM (64/256/1024QAM) cable modem systems. In wire‐lined cable systems, it seems that the cable channel may display a nearly invariant multi‐path effect over a considerable time, and then the blind equalizer can have the capability to recover the received high‐order QAM symbols on wired cables. The proposed blind equalization algorithm applies a two‐stage convergence scheme, which results from the multi‐modulus algorithm (MMA), sometimes called the modified constant modulus algorithm (MCMA), and the DD LMS‐like method. In the first equalization stage, the joint MMA and DD LMS‐like methods are applied for the purpose of fast convergence. When the convergence process reaches an acceptably steady condition, the convergence detector changes the process into the second stage. In the second equalization stage, the enhanced DD LMS‐like method with the generalized multi‐modulus is applied to reduce the mean square error (MSE) of equalizations further. For high‐order QAM cable systems, the proposed method performs faster than the previous well‐known blind methods do. Simultaneously, the proposed algorithm also provides less MSE and lower symbol error rate (SER) than the other blind methods do at the same signal‐to‐noise ratio (SNR).

QAM Symbol Error Rate in OFDM Systems Over Frequency-Selective Fast Ricean-Fading Channels

For digital data transmission using an orthogonal frequency-division multiplexing (OFDM) over fading channels, the interchannel- interference (ICI) term caused by the Doppler spread is correlated with the desired signal term. Nonetheless, many researchers have assumed the ICI term to be uncorrelated with the desired signal term. In this paper, we present the symbol-error-rate (SER) performance for the M-quadratic amplitude-modulation OFDM systems over Ricean fading using the exact correlated ICI model. The results are then compared with the uncorrelated ICI results. We find that the two results present some differences. The differences tend to increase as the Ricean factor is increased.

Effect of the Phase/Quadrature Error and I/Q Gain Imbalance for QAM Symbol Error Probability

In this letter, we derive an analytical expression for computing the symbol error probability (SEP) of the M-ary quadrature amplitude modulation (M-QAM) in the joint presence of phase/quadrature error and VQ gain imbalance over an additive white Gaussian noise (AWGN) channel. The derived expression containing only the two-dimensional Gaussian Q-function can be used to compute the SEP of M-QAM in various fading channels by making use of the moment-generating function (MGF) approach.

A systematic bit-wise decomposition of M-ary symbol metric

In this paper, we present a systematic recursive formula for bit-wise decomposition of M-ary symbol metric. The decomposed bit metrics can be applied to improve the performance of a system where the information sequence is binary-coded and interleaved before M-ary modulated. A traditional receiver designed for certain system is to de-map the received M-ary symbol into its binary isomorphism so as to facilitate the subsequent bit-based manipulation, such as hard-decision decoding. With a bit-wise decomposition of M-ary symbol metric, a soft-decision decoder can be used to achieve a better system performance. The idea behind the systematic formula is to decompose the symbol-based maximum-likelihood (ML) metric by equating a number of specific equations that are drawn from squared-error criterion. It interestingly yields a systematic recursive formula that can be applied to some previous work derived from different standpoint. Simulation results based on IEEE 802.11a/g standard show that at bit-error-rate of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> , the proposed bit-wise decomposed metric can provide 3.0 dB, 3.9 dB and 5.1 dB improvement over the concatenation of binary-demapper, deinterleaver and hard-decision decoder respectively for 16QAM, 64QAM and 256QAM symbols, in which the in-phase and quadrature components in a complex M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> -QAM symbol are independently treated as two real M-PAM symbols. Further empirical study on system imperfection implies that the proposed bit-wise decomposed metric also improves the system robustness against gain mismatch and phase imperfection. In the end, a realization structure that avails the recursive nature of the proposed bit-decomposed metric formula is addressed

A MIMO System with Backward Compatibility for OFDM-Based WLANs

Orthogonal frequency division multiplexing (OFDM) has been selected as the basis for the new IEEE 802.11a standard for high-speed wireless local area networks (WLANs). We consider doubling the transmission data rate of the IEEE 802.11a system by using two transmit and two receive antennas. We propose a preamble design for this multi-input multi-output (MIMO) system that is backward compatible with its single-input single-output (SISO) counterpart as specified by the IEEE 802.11a standard. Based on this preamble design, we devise a sequential method for the estimation of the carrier frequency offset (CFO), symbol timing, and MIMO channel response. We also provide a simple soft detector based on the unstructured least square approach to obtain the soft information for the Viterbi decoder. This soft detector is very simple since it decouples the multidimensional QAM symbol detection into multiple one-dimensional QAM symbol-and further PAM symbol-detections. Both the sequential parameter estimation method and the soft detector can provide excellent overall system performance and are ideally suited for real-time implementations. The effectiveness of our methods is demonstrated via numerical examples.